Industrial vehicles, such as fork lift trucks, are typically driven in a reverse direction and frequently make tight turns. The conventional steering mechanism of such vehicles utilize the vehicle's rear wheels for turning the vehicle in a left or right direction and oftentimes incorporate hydraulic power steering to reduce the effort required by the driver to steer the vehicle. In particular such systems utilize a hydraulic rear axle cylinder.
FIG. 5 illustrates a rear axle cylinder 51 that has a hydraulic fluid filled cylinder tube 52 having gray cast iron rod guides 53 and 54 attached at both ends. A piston rod 55, supported by rod guides 53 and 54 in the cylinder tube 52, reciprocates in a rightward and leftward direction. Both end portions of the piston rod 55 are coupled to the vehicle's rear wheels via tie rods, knuckles or the like (not shown). Upon movement of the piston rod 55 leftward, the piston's motion is converted to the turning motion of the rear wheels via the tie rods, knuckles, etc.
Pressure chambers 57 and 58 are defined in the cylinder tube 52 by a piston 56 mounted on the outer surface of the piston rod 55. An oil inlet port 59 formed in the left rod guide 53 is connected to a hydraulic pump via a piping 60, and to the pressure chamber 57 via an oil passage 61. Likewise, an oil inlet port 62, formed in the right rod guide 54, is connected to the hydraulic pump via a piping 63 and to the pressure chamber 58 via an oil passage 64. When the hydraulic pump supplies hydraulic fluid to the pressure chamber 57 via the combination of the piping 60, the oil inlet port 59 and the oil passage 61, the hydraulic fluid in the pressure chamber 58 is discharged via the oil passage 64, the oil inlet port 62 and the piping 63. This causes the rightward movement of the piston rod 55.
Each of the oil passages 61 and 64 is formed having a vertical hole 65, an oblique hole 66 and an annular groove 67. The vertical hole 65 extends inward in the radial direction of the rod guide 53 or 54 from the oil inlet port 59 or 62. The annular groove 67 is formed entirely in the inner wall of each of the rod guides 53 and 54. The oblique hole 66 is formed in each of the rod guides 53 and 54, and connects the vertical hole 65 to the annular groove 67. This structure is employed both to widen as much as possible the area of portions C where the piston rod 55 slides against the rod guides 53 and 54, and to make the rod guides 53 and 54 more compact.
More specifically, if the oil passages 61 and 64 are formed each having only the vertical hole 65 and the annular groove 67, as indicated by the broken lines in FIG. 5, the surface area of the portions C where the piston rod 55 slides against the rod guides 53 and 54 decreases. Accordingly, when a force is applied to the piston rod 55 from the tie rod in a direction perpendicular to the moving direction of the piston rod 55 (i.e. an up and down direction), such a force cannot easily be distributed over the decreased slide-contact portion C. This impairs the smooth movement of the piston rod 55 in the cylinder tube 52.
One solution to this problem, is to increase the thicknesses of rod guides 53 and 54 (in the right and left direction) in order to widen the areas of the slide-contact portions C. This however enlarges the rod guides 53 and 54, and makes the rear axle cylinder 51 larger and heavier.
According to the above prior art, the oblique holes 66 are formed midways along the oil passages 61 and 64 in order to increase the surface area of the contact portions C without making the rod guides 53 and 54 thicker. Accordingly, when a force perpendicular to the moving direction of the piston rod 55 is applied to the rod 55, the force is easily distributed over the slide-contact portion C. This allows the piston rod 55 to slide smoothly, in the cylinder tube 52 without making the rear axle cylinder 51 larger or heavier.
Currently, during the manufacture of the rod guides 53 and 54 which incorporate the oil passages 61 and 64, it is relatively easy to form vertical hole 65 and annular groove 67. On the other hand, it is relatively difficult to form the oblique hole 66 in such a way that accurately aligns and links the hole 66 to the vertical hole 65. Conventional formation of the oblique hole 66 requires complex steps and high-precision processing technology. This complicates the production of the rear axle cylinder 51 and increases the manufacturing cost.
Further, the rod guides 53 and 54 should satisfy the following conditions:
(1) Although rod guides 53 and 54 are in direct contact with the piston rod 55, it is generally undesirable that such contact either damage or cause premature wearing of the sliding piston rod 55. The rod guides 53 and 54 should therefore be made of a material having a lower rigidity and smaller frictional coefficient than the piston rod 55.
(2) To support the piston rod 55 in a slidable manner, the rod guides 53 and 54 should have a sufficient strength.
The conventional material used for constructing rod guides 53 and 54, i.e., gray cast iron, however cannot satisfy the requirements of the first and second conditions. Consequently, the piston rod 55 is likely to be damaged or its contact portions easily worn out by the sliding motion of piston rod 55. That is, the conventional piston rod 55 does not exhibit good durability characteristics.
The present invention has been accomplished with a view to solving the above disadvantages. It is an object of the present invention to provide a hydraulic cylinder incorporating a piston rod with enhanced durability characteristics in order to decrease the damage or wear to the piston rod caused by its sliding motion and to decrease the costs associated with the manufacturing of the hydraulic cylinder.